We have studied the effect of various inhibitors and of temperature on surface topology of mouse lymphocytes macrophages and mast cells. The mast cell surface proved very resistant to effects of these agents. Inhibitors of energy metabolism are promoters of microvillus extension on lymphocytes and macrophages. Local anaesthetics and glutathione oxidizing agents (viz. diamide) are inhibitors of microvillus extension. Low temperature affects surface topology and over-rides the effect of antimetabolites. Low temperature and diamide reciprocally compete; in the presence of both, the cell surface appears normal. These promoters and inhibitors provide a way to examine the control of cell surface topology. Double fixation of cells in glutaraldehyde and osmium tetroxide induces blebs in cell membranes. No remedy for this artifact has been discovered. BIBLIOGRAPHIC REFERENCES: Orenstein, J.M., Johnson, L., Shelton, E., and Lazzarini, R.A.: The shape of vesicular stomatitis virus. Virology 71: 291-301, 1976. Orenstein, J.M., and Shelton, E.: Surface topography and interactions between mouse peritoneal cells allowed to settle on an artificial substrate: Observations by scanning electron microscopy. Exp. Mol. Path. 24, 201-219, 1976.